Shared patterns of segment size development in trilobites and vertebrates.

The article investigates the developmental control of segment size patterning in trilobites, an extinct group of arthropods, to test whether an inhibitory cascade (IC) model—previously established in vertebrates—also governs segment development in arthropods. Analyzing 128 trilobite species and ontogenies of three species, the study finds strong linear size patterning of newly generated segments in the pygidium (segment addition zone), consistent with IC-like regulation, while adult thoracic segments show more variable patterns influenced by differential growth rates. Comparative data from the stem arthropod Anomalocaris and extant insects reveal similar IC-like linear segment size patterns during early development, suggesting this mechanism is a common default mode of segment development across arthropods. The findings imply that such developmental biases have shaped long-term morphological evolution in trilobites and potentially other arthropods, paralleling patterns observed in vertebrates.